Mwadham M. Kabanda

Experimental and theoretical studies on the corrosion inhibition of mild steel by some sulphonamides in aqueous HCl

Corrosion inhibition studies of mild steel in aqueous HCl by some sulphonamides namely sulphamethazine (SMT), sulphachloropyridazine (SCP), sulphabenzamide (SBZ) and sulphaquinoxaline (SQX) has been investigated using experimental techniques (such as weight loss, potentiodynamic polarization (PDP), Electrochemical Impedance Spectroscopy (EIS), Fourier transform infrared spectroscopy (FTIR) and Scanning Electron Microscopy (SEM)) and theoretical methods (using the Density Functional Theory (DFT)). All the compounds effectively inhibited the corrosion process by becoming adsorbed on the metal surface following the Langmuir adsorption isotherm model. The electrochemical results showed that these inhibitors are mixed-type. The theoretical studies were undertaken to provide mechanistic insight into the roles of the different substituents on the corrosion inhibition and adsorption behaviour of the studied compounds. The calculated quantum chemical parameters include the highest occupied molecular orbital (HOMO), the energy of the HOMO, dipole moment and partial atomic charges, etc. The calculated molecular properties were compared across the structures of the four compounds in order to identify trends related to their reactivity and their corrosion inhibition ability. The results also show that the ability of the sulphonamides to inhibit metal corrosion is strongly dependent on the electron donating ability of the substituent group and that the preferred site for interaction with the metal surface, in all the sulphonamides, is the SO2 group.

A Computational Study of the Effects of Different Solvents on the Characteristics of the Intramolecular Hydrogen Bond in Acylphloroglucinols

Acylphloroglucinols are a broad class of compounds, derivatives from 1,3,5-trihydroxybenzene, and exhibiting a variety of biological activities. They are characterized by the presence of at least one COR group, whose sp(2) O can form an intramolecular hydrogen bond with a neighboring phenolic OH. This H-bond plays dominant roles in determining conformational preferences and energy, and is expected to play significant roles in biological activity mechanisms, which strongly motivates the study of its characteristics in solution. A computational study of a representative number of actual and model structures with different R was carried out in three solvents with different polarities and different types of interactions with solute molecules: water, acetonitrile, and chloroform, utilizing the PCM model. Calculations were mostly performed at the HF/6-31G(d,p) level because of affordability reasons in view of the size and number of the structures considered (the smallest structures were also calculated at MP2/6-31+G(d,p) level). Comparison with the results of a previous study in vacuo shows similar patterns within each medium, pointing to similarities in the influence of relevant geometry factors on the characteristics of the H-bond. The medium appears to have little influence on the parameters of the H-bond. Comparison across media of the energy increase on H-bond removal (an indication of the H-bond strength) is complicated by the greater solvent stabilization of the conformer resulting from H-bond removal, with respect to the one in which the H-bond is present. Several factors, however, would point to a strength not too different from that observed in vacuo.

Synthesized photo-cross-linking chalcones as novel corrosion inhibitors for mild steel in acidic medium: experimental, quantum chemical and Monte Carlo simulation studies

New chalcone derivatives namely (E)-(1-(5-(4-(3-(4-methylphenyl)-3-oxoprop-1-enyl)phenoxy)pentyl)-1H-1,2,3-triazol-4-yl)methyl acrylate (CH-5), (E)-(1-(5-(4-(3-(4-methylphenyl)-3-oxoprop-1-enyl)phenoxy)hexyl)-1H-1,2,3-triazol-4-yl)methyl acrylate (CH-6) and (E)-(1-(5-(4-(3-(4-methylphenyl)-3-oxoprop-1-enyl)phenoxy)decyl)-1H-1,2,3-triazol-4-yl) methyl acrylate (CH-10) were synthesized and characterized by Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopic techniques. Ultraviolet-visible (UV-vis) spectra of the synthesized compounds confirmed that the chalcones undergo photo-cross-linking upon irradiation with UV-light. Potentiodynamic polarization measurements showed that both the intact and photo-cross-linked chalcones are mixed-type corrosion inhibitors for mild steel in aqueous hydrochloric acid. The EIS results showed an increase in charge transfer resistance with increasing concentration of the inhibitors. The chalcone derivatives adsorb spontaneously on the mild steel surface and their adsorption obeyed the Langmuir adsorption isotherm. The adsorption mode revealed the possibility of competitive physisorption and chemisorption mechanisms. Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX) analyses confirmed that the chalcones formed a protective film on the mild steel surface. The overall results showed that the photo-cross-linked chalcones are better corrosion inhibitors than the intact chalcones. The results of quantum chemical calculations and Monte Carlo simulation studies are in good agreement with experimental results.

Preferential alkali metal adduct formation by cis geometrical isomers of dicaffeoylquinic acids allows for efficient discrimination from their trans isomers during ultra-high-performance liquid chromatography/quadrupole time-of-flight mass spectrometry.

RATIONALEnCaffeoylquinic acid (CQA) derivatives are a group of structurally diverse phytochemicals that have attracted attention due to their many health benefits. The structural diversity of these molecules is due in part to the presence of regio- and geometrical isomerism. This structural diversity hampers the accurate annotation of these molecules in plant extracts. Mass spectrometry (MS) is successfully used to differentiate between the different regioisomers of the CQA derivatives; however, the accurate discrimination of the geometrical isomers of these molecules has proven to be an elusive task.nnnMETHODSnUV-irradiated methanolic solutions of diCQA were analyzed using an ultra-high-performance liquid chromatography/quadrupole time-of-flight mass spectrometry (UHPLC/QTOFMS) method in negative ionisation mode. An in-source collision-induced dissociation (ISCID) method was optimized by varying both the capillary and cone voltages to achieve differential fragmentation patterns between UV-generated geometrical isomers of the diCQAs during MS analyses.nnnRESULTSnChanges in the capillary voltage did not cause a significant difference to the fragmentation patterns of the four geometrical isomers, while changes in the cone voltage resulted in significant differences in the fragmentation patterns. The results also show, for the first time, the preferential formation of alkali metal (Li(+), Na(+) and K(+)) adducts by the cis geometrical isomers of diCQAs, compared to their trans counterparts.nnnCONCLUSIONSnOptimized QTOFMS-based methods may be used to differentiate the geometrical isomers of diCQAs. Finally, additives such as metal salts to induce adduct formation can be applied as an alternative method to differentiate closely related isomers which could have been difficult to differentiate under normal MS settings.

Porphyrins as Corrosion Inhibitors for N80 Steel in 3.5% NaCl Solution: Electrochemical, Quantum Chemical, QSAR and Monte Carlo Simulations Studies

The inhibition of the corrosion of N80 steel in 3.5 wt. % NaCl solution saturated with CO2 by four porphyrins, namely 5,10,15,20-tetrakis(4-hydroxyphenyl)-21H,23H-porphyrin (HPTB), 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphyrin (T4PP), 4,4′,4″,4‴-(porphyrin-5,10,15,20-tetrayl)tetrakis(benzoic acid) (THP) and 5,10,15,20-tetraphenyl-21H,23H-porphyrin (TPP) was studied using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, scanning electrochemical microscopy (SECM) and scanning electron microscopy (SEM) techniques. The results showed that the inhibition efficiency, η% increases with increasing concentration of the inhibitors. The EIS results revealed that the N80 steel surface with adsorbed porphyrins exhibited non-ideal capacitive behaviour with reduced charge transfer activity. Potentiodynamic polarization measurements indicated that the studied porphyrins acted as mixed type inhibitors. The SECM results confirmed the adsorption of the porphyrins on N80 steel thereby forming a relatively insulated surface. The SEM also confirmed the formation of protective films of the porphyrins on N80 steel surface thereby protecting the surface from direct acid attack. Quantum chemical calculations, quantitative structure activity relationship (QSAR) were also carried out on the studied porphyrins and the results showed that the corrosion inhibition performances of the porphyrins could be related to their EHOMO, ELUMO, ω, and μ values. Monte Carlo simulation studies showed that THP has the highest adsorption energy, while T4PP has the least adsorption energy in agreement with the values of σ from quantum chemical calculations.

Adsorption and Corrosion Inhibition Studies of Some Selected Dyes as Corrosion Inhibitors for Mild Steel in Acidic Medium: Gravimetric, Electrochemical, Quantum Chemical Studies and Synergistic Effect with Iodide Ions

The corrosion inhibition properties of some organic dyes, namely Sunset Yellow (SS), Amaranth (AM), Allura Red (AR), Tartrazine (TZ) and Fast Green (FG), for mild steel corrosion in 0.5 M HCl solution, were investigated using gravimetric, potentiodynamic polarization techniques and quantum chemical calculations. The results showed that the studied dyes are good corrosion inhibitors with enhanced inhibition efficiencies. The inhibition efficiency of all the studied dyes increases with increase in concentration, and decreases with increase in temperature. The results showed that the inhibition efficiency of the dyes increases in the presence of KI due to synergistic interactions of the dye molecules with iodide (I−) ions. Potentiodynamic polarization results revealed that the studied dyes are mixed-type inhibitors both in the absence and presence of KI. The adsorption of the studied dyes on mild steel surface, with and without KI, obeys the Langmuir adsorption isotherm and involves physical adsorption mechanism. Quantum chemical calculations revealed that the most likely sites in the dye molecules for interactions with mild steel are the S, O, and N heteroatoms.

Antioxidant Radical Scavenging Properties of Phenolic Pent-4-En-1-Yne Derivatives Isolated From Hypoxis Rooperi. A DFT Study in vacuo and in Solution

B3LYP/6-311+G* calculations were performed on five phenolic pent-4-en-1-yne derivatives, namely rooperol, dehydroxyrooperol I and II, bis-dehydroxyrooperol, and hypoxoside, to investigate and compare their antioxidant radical scavenging properties. The main objectives were to model, for the first time, the antioxidant properties of dehydroxyrooperol I and II and bis-dehydroxyrooperol, to clarify the possible role of hypoxoside as radical scavenger and to provide insight into molecular geometry factors influencing the radical scavenging properties of phenolic pent-4-en-1-yne derivatives. The study was conducted by checking the molecules’ ability for two main antiradical mechanisms, hydrogen atom transfer and electron transfer. The results indicated that the antiradical properties of rooperol and the two dehydroxyrooperols are predominantly exerted through the hydrogen atom transfer mechanism and are the outcome of the stabilization of their neutral radical species through the interplay of hydrogen bond(s) and spin density distributions stabilizing their neutral radical species. The antiradical properties of bis-dehydroxyrooperol may be explained only in terms of the spin density distribution of its neutral radical species. The antiradical properties of hypoxoside, with O−H bond dissociation enthalpy considerably greater than that of phenol, could be considered to be minor and mainly governed by electron transfer mechanism.

Conformational, electronic and antioxidant properties of lucidone, linderone and methyllinderone: DFT, QTAIM and NBO studies

Theoretical studies on lucidone, linderone and methyllinderone were performed to investigate factors that contribute to structural stability and to elucidate the antioxidant properties and mechanisms. The study was performed in different media utilising the density functional theory with different functionals and the 6-311+ G(d,p) basis set. The antioxidant activity has been considered through the electron transfer and metal chelation mechanisms. The results show that the stability of the tautomers and conformers is due to the presence of several intramolecular hydrogen bonds. The ionisation potential values suggest that the antiradical activity increases with the increase in the number of OCH3 groups substituted on the cyclopentene-1,3-dione ring. In vacuo, the spin density of the Fe(II) cation upon ligand coordination decreases to 3.0−3.5, whereas the ligand spin density approaches 1, indicating that it is oxidised to a radical cation. The metal ion affinity (MIA) is influenced by the position and number of OCH3 substituted on the acylcyclopentene-1,3-dione ring. A very favourable MIA, in vacuo, is obtained when Fe(II) is chelated between the sp2 O and sp3 O atoms. An estimation of MIA in an aqueous solution shows a remarkable decrease with respect to the results in vacuo.

Structures, Stabilization Energies, and Binding Energies of Quinoxaline···(H2O)n, Quinoxaline Dimer, and Quinoxaline···Cu Complexes: A Theoretical Study

Quinoxaline is a parent structure for a broad class of N-heteroaromatic compounds, many of which exhibit various biological activities. The interaction of quinoxaline with explicit water molecules or metal ions and the formation of quinoxaline dimer play an important role in many of the biological activities of quinoxaline. This study investigates the structures, stabilization, and binding energies of quinoxaline complexes with water, transition metal ions, and quinoxaline dimer to provide information on the preferred geometries, interaction energies, and type of noncovalent interactions accounting for the stability of the complexes. The investigations are performed in vacuo and in water solution using MP2 and DFT methods. The results of the study on the quinoxaline···(H(2)O)(n) show that the preferred adducts in vacuo involve one, two, or three water molecules hydrogen bonded to the N atom and the neighboring H atom of the C(sp2)-H group. The results in water solution show a preference for water-water clustering. The dimers of quinoxaline are stabilized by either π-π stacking or weak C-H···N intermolecular hydrogen bonds. The relative stability of the quinoxaline···Cu complexes depends on the site on which the Cu ion binds and the binding strength depends on both the nature of the cation and the binding site.

Some Phthalocyanine and Naphthalocyanine Derivatives as Corrosion Inhibitors for Aluminium in Acidic Medium: Experimental, Quantum Chemical Calculations, QSAR Studies and Synergistic Effect of Iodide Ions

The effects of seven macrocyclic compounds comprising four phthalocyanines (Pcs) namely 1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (Pc1), 2,3,9,10,16,17,23,24-octakis(octyloxy)-29H,31H-phthalocyanine (Pc2), 2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine (Pc3) and 29H,31H-phthalocyanine (Pc4), and three naphthalocyanines namely 5,9,14,18,23,27,32,36-octabutoxy-2,3-naphthalocyanine (nPc1), 2,11,20,29-tetra-tert-butyl-2,3-naphthalocyanine (nPc2) and 2,3-naphthalocyanine (nP3) were investigated on the corrosion of aluminium (Al) in 1 M HCl using a gravimetric method, potentiodynamic polarization technique, quantum chemical calculations and quantitative structure activity relationship (QSAR). Synergistic effects of KI on the corrosion inhibition properties of the compounds were also investigated. All the studied compounds showed appreciable inhibition efficiencies, which decrease with increasing temperature from 30 °C to 70 °C. At each concentration of the inhibitor, addition of 0.1% KI increased the inhibition efficiency compared to the absence of KI indicating the occurrence of synergistic interactions between the studied molecules and I− ions. From the potentiodynamic polarization studies, the studied Pcs and nPcs are mixed type corrosion inhibitors both without and with addition of KI. The adsorption of the studied molecules on Al surface obeys the Langmuir adsorption isotherm, while the thermodynamic and kinetic parameters revealed that the adsorption of the studied compounds on Al surface is spontaneous and involves competitive physisorption and chemisorption mechanisms. The experimental results revealed the aggregated interactions between the inhibitor molecules and the results further indicated that the peripheral groups on the compounds affect these interactions. The calculated quantum chemical parameters and the QSAR results revealed the possibility of strong interactions between the studied inhibitors and metal surface. QSAR analysis on the quantum chemical parameters obtained with B3LYP/6-31G (d,p) method show that a combination of two quantum chemical parameters to form a composite index provides the best correlation with the experimental data.